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394                         METALLURGY OF IRON AND STEEL.
rails at a slightly higher price, and then start up the Bessemer plant on other orders and let the open-hearth lie idle.
(5) It may seem possible to have a number of mills and have the open-hearth and. Bessemer plants both operating continuously and distributing their product as orders demand. One or two works in the country are able to do this to a greater or less extent, but it is impossible to do it and maintain the proper coordination of dependent factors and keep the operating costs in each department at a minimum.
We may conclude, therefore, that small lots of open-hearth rails may be made, but their production on a large scale'means a plant laid out with that end in view, and if this plant is not guaranteed a regular line of business extending over many years at an increased price, it will be a losing venture.
In the case of structural shapes there is no difficulty in obtaining all needed sections in open-hearth steel, and it should be used in all structures, like railroad bridges, where the metal is under constant shock. The method by which the steel is made cannot be discovered by ordinary chemical analysis. Certain experiments indicate that there is a difference between Bessemer and open-hearth steel in the character of the occluded gases, but it is doubtful if any expert would risk his reputation by asserting positively, from any such evidence, that a certain steel was made by either one or the other process.
SEC. XYIIIb.—Chemical specifications.—Another point concern-• ing which there is room for discussion is the propriety of limiting the chemical* composition.    Some engineers contend that, if the physical tests are fulfilled, the making of the metal is an entirely foreign matter.   This position is untenable, for it would be possible to make a steel with 0.25 per cent, of phosphorus which would satisfy the ordinary tests of strength and ductility, and although such a content could usually be detected in the shops, a considerable proportion of the bars might pass muster.   It is impossible to fix a limit of phosphorus below which there is no danger of treacherous breakage, but it is certain that, as the content is reduced, the danger of disaster disappears.    On this account it becomes the duty of the engineer to specify the composition of the metal that he buys. In ordinary roof-trusses and similar work there is no necessity 'ifor stringency, and Bessemer steel with a maximum content of .10